And now we turn to one of the strangest spy stories we've ever heard. A man is coming out of the shadows tonight saying he helped the united states snare one of the most dangerous al qaeda terrorists in the world. How? Abc's chief investigative correspondent brian ross with the spy, the terrorist and the wife who may have led the terrorist to a mistake. Reporter: He evaded the u.S. For years. But a new report today says anwar al-awlaki's downfall may have been linked to his wife's shopping list for pantene sham me shampoo and nivea deodorant. We have sent other people before and nothing really suited her tas taste, al awlaki wrote, as he sent the list to someone he thought was a trusted al qaeda aide. But the trusted aide now says he was a spy. In an interview with a danish newspaper, morten storm, a danish covert to islam, says he worked with the c.I.A and danish kill al-awlaki. Translator: He was a terrorist. He wanted to kill innocent people. Reporter: Storm himself was originally part of a radical islamic group in europe. This was him in 2005 at an anti-u.S. Rally that made news in london. Six months later he says he had been recruited by danish intelligence for his dangerous role as a double agent. If al qaeda had come to the conclusion that he was spying for the danes and the cia, they would have killed him immediately. Reporter: In fiction like the popular "homeland" tv show, spies and agents working against terrorists use all kinds of high-tech devices to track their targets. In the case of al-awlaki, the danish spy says the cia bugged a thumb drive he used to send messages including his wife's shopping list. S. Translator: A transmitter would be planted in usb flash or a program that could send a message to a tower, so he could be localized. Reporter: Al-awlaki was killed by a u.S. Drone strike on , what american offic.I.Als the u.S. Was using a variety of ways to track him and the cia declined to offer today any comment on the role of the alleged involvement by the danish spy. Okay, brian, thank you. After our break right here on

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